Code simplification; improve muse-list-item-type function.

Code simplification; improve muse-list-item-type function.

* lisp/muse.el (muse-list-item-type): Disambiguate between 'dl-term and
  'dl-entry, instead of just returning 'dl.
  (muse-forward-dl-term, muse-forward-dl-entry): Use
  muse-forward-list-item.
  (muse-forward-dl-part): Remove, merge functionality into
  muse-forward-list-item.
  (muse-forward-list-item): Add a few conditions needed for skipping
  nested 'dl-term and 'dl-entry items.

--This line, and those below, will be ignored--
Files to commit:
   lisp/muse.el
   lisp/muse-mode.el

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git-archimport-id: mwolson@gnu.org--2006/muse--main--1.0--patch-46